THE MUSCULAR SYSTEM 191
tendons of the extensor communis to the fourth and fifth digits.
The extensor indicis is inserted similarly in ilan
and the Gorilla ; in the Chimpanzee it may also supply the middle finger; in the Orang it supplies the raedius and annularis, and it supplies the medius, index and annularis in the Gibbons.
Extensors of the Pollex.—In the Chimpanzee the extensor ossis metacarpl pollicis arises from the upper two-thirds of the mesial border of the radius; and its broad tendon is inserted into the trapezium and thumb sesamoid. The extensor primi internodii pollicis rises from the upper third of the lateral border of the ulna; its slender tendon goes to the base of the first meta-carpal bone. In the Gorilla the extensor ossis meta-carpi pollicis goes to the first metacarpal bone, but it goes to the pollical sesamoid bone in the Gibbon and Orang. In the Gorilla the extensor primi internodii is attached by a slip to the first phalanx of the pollex.
The extensor secundi internodii pollicis in the Chimpanzee arises from the third quarter of the shaft of the ulna below the extensor primi internodii pollicis and above the extensor indicis. It is inserted, according to Hepburn (83), into the base of the first phalanx, but Humphry (340), Macalister (350), Vrolik (411), Wilder (414), Wyman (417), and Sonntag (399) give its insertion as the ungual phalanx. In many Apes it sends a slip to the first phalanx, thus replacing the extensor primi internodii pollicis.
Interossei.—In the Apes and Man there are four dorsal interossei abducting the fingers from a line drawn through the medius, and in all except the Chimpanzee